The present application has been divided out from Ser. No. 964,790 which relates to a device for the retrieval of ocean bottom seismic cable.
Seismic investigations at sea are usually carried out by using surface seismic, where a seismic cable with monitoring equipment is towed behind a vessel, where, for example, air is "shot" into the water and forms sound waves which are reflected by the layers in the underlying rock formation, and where these reflections are registered by means of the monitoring equipment of the seismic cable.
Computer analyses of the registered pattern of reflections provide a basis for mapping structures of interest below the ocean floor.
Ocean bottom seismic bears a strong resemblance to surface seismic, with the exception that the monitoring cables are positioned on the ocean floor, so that the monitoring equipment is in direct contact with the floor. The advantage of this type of seismic is that by direct contact between the underlying floor and the monitoring equipment the sensitivity of the equipment is increased, and the measurements obtained are more accurate and detailed, giving the basis for an even more detailed mapping of the underlying structures. However, ocean bottom seismic is more expensive to carry out than surface seismic.
Typically, an ocean bottom seismic cable is 10 to 12 km in length and is composed of a plurality of sections of about 300 m in length fitted together using a connecting piece, and the monitoring equipment which is to record signals from the ocean floor is placed at defined positions on the cable.
The cable can be laid on the ocean floor by being released from the stern of a vessel travelling at a speed of 2 to 5 knots across the ocean floor, upon which the monitoring cable is to be placed. During the registration of ocean bottom seismic a plurality of cables are laid in parallel relation and spaced apart at a given distance on the ocean floor. Normally, the work will be carried out continuously so that two to four cables are used for monitoring, whilst the monitoring field is moved across the ocean floor by taking up the cable which lies outermost in the monitoring field, and moving this cable parallel with the other cables across to the other side of the monitoring field. During an operation of this kind three vessels are normally involved, one vessel which "shoots" and two vessels which alternate between monitoring the laid cables and moving the monitoring field by taking up a cable along one side edge of the field and moving across to the other side edge thereof.
Today, the ocean bottom seismic cable is retrieved by running the cable over a wheel which is located on the side at the front of the vessel. The cable is led over a wheel and in between a powered rubber wheel which ensures that the cable is passed abaft on the boat in a groove to the stern, where a block, suspended in a travelling crane, lays the cable out across the deck so as to facilitate the location of the specific read-off points and the connecting pieces. The cable thus lies like a "heap of spaghetti" on the deck, where only the said read-off points and the connecting pieces are accessible for measurement. En route to and from the survey, there may be two to three such cables lying on top of one another on a deck of a vessel of this kind.
A major and costly problem in connection which such ocean bottom seismic is that the cable tends to incur some damage, and much time is spent repairing this. This repair work is usually carried out by locating the fault through measurements at the defined read-off points and then replacing one or more sections. With the cable lying in a heap on deck, a replacement of this kind is difficult and extremely time-consuming.
Studies have also shown that about 50% of the damage to the cable takes place on board the boat during retrieval and deployment. The traditional manner of deploying the cable is that the said travelling crane and block are positioned on the stern of the boat, so that the cable is led out through this block when being deployed. The cable, which is lying across the deck, is pulled during normal deployment at a speed of up to 2.5 m per second, which means that the cable which is swung from side to side on the deck will be thrown against other cables and against the boat during deployment. The monitoring equipment found on the cable is delicate equipment and can easily be damaged by such buffeting, and this requires the replacement of sections when the cables are redeployed. Moreover, the stern of the ship during a deployment operation of this kind will be a dangerous area, to which access is only possible with stringent safety measures.
Damage to the cable occurs also because of jerks and uneven pull on the cable during the retrieval thereof. Normally, the cable runs, as mentioned above, over a wheel at the front on the side of the vessel. This wheel is fixed and follows the motions of the vessel, so that in rough seas there will be jerks on the cable. Moreover, the cable must run straight up from the ocean bottom, so that there is no tension in any direction along the ocean bottom, as this could cause the cable to become caught on objects on the bottom.
A preferred device for the retrieval of seismic cable is described in Ser. No. 964,790, where the cable is pulled up from the bottom and hauled aboard a vessel over a roller, a wheel or the like, wherein the device comprises two or more wheels and where the cable is first led over a front wheel located on an arm, which arm is pivotally mounted about an axis, and that the cable is then passed under a second wheel before it is brought aboard the vessel, optionally via one or more additional wheels.